Ibrance's performance over the three month period beat expectations, boosted by continued adoption among U.S. physicians. Revenue from fourth quarter sales hit $643 million, roughly double the total from the same period last year and up 17% on a sequential quarter basis.

Company CEO Ian Read pointed to Ibrance as a "significant growth driver" for Pfizer and emphasized his confidence the drug can remain a leader in the CDK 4/6 inhibitor class of drugs despite looming competitive threats from Novartis and Eli Lilly.

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Ibrance's success has already propelled the drug to become Pfizer's fourth best-selling medicine after Prevnar, Lyrica and Enbrel franchises.

To date, almost all of Ibrance's sales have come from the U.S. market, where Pfizer estimates it has a 45% market share in first-line treatment of HR+/HER2- metastatic breast cancer.

Pifzer won approval for Ibrance in the EU last November, however, and expects to launch across Europe throughout 2017, likely adding further momentum behind the drug this year.

But the company is also hoping to expand Ibrance use among physicians in the U.S., particularly following the publication of strong results from a Phase 3 confirmatory trial last November (Ibrance was originally approved on an accelerated basis). Pfizer expects this additional data to help convince late-adopting doctors to prescribe Ibrance.

Already approximately 9,500 physicians have prescribed Ibrance to roughly 50,000 patients in the U.S., said Albert Bourla, head of Pfizer's Innovative Health unit, on Tuesday's earnings call. Those numbers are up from the third quarter, when Pfizer reported 8,500 physicians prescribing to about 40,000 patients.

"Moving forward, we believe the growth will come from late adopters, many of whom have already prescribed Ibrance but in a limited number of patients," Bourla said.

"Twenty percent of the approximately 12,000 prescribing physicians in the U.S. have not yet prescribed the product."

Winning over those doctors and expanding market share will be important as Pfizer may soon face competition from new entrants in the CDK 4/6 class.

Novartis' application for approval of ribociclib, its CDK 4/6 inhibitor, was accepted by the Food and Drug Administration in November 2016 — putting a potential approval in the April timeframe. Results from a Phase 3 study showed strong efficacy, underpinning forecasts of blockbuster sales.

Eli Lilly also has a potential competitor, abemaciclib, moving through development, although a filing with the FDA isn't expected until 2018.